Hydraulic dredging provides an economical way of maintaining adequate depth in navigation channels necessary for commercial shipping. While hydraulic dredges differ in design and size, depending on the particular application intended, a typical hydraulic cutterhead dredge comprises a barge, a downwardly inclined ladder extending from the front of the barge, a rotating cutter at the lower end of the ladder for cutting and agitating material in the channel bottom, a main pump on the barge, and a suction pipe extending from the main pump and down the ladder, the suction pipe having an inlet, called the suction mouthpiece, proximate the cutter so that material stirred up by the cutter is drawn into the pipe by the main pump. The ladder is typically positioned by a ladder suspension system including a vertical H-frame extending upwardly from the front of the barge, an inclined A-frame extending forwardly and upwardly from the bow of the barge, and suitable rigging. A motor at the top of the ladder transmits rotary motion to the cutter by means of a cutter shaft extending down the ladder. Positioning and movement of the dredge is accomplished with paired vertical spuds located at the stern of the dredge along with a winch located on the forward deck. This winch controls wire ropes that pass through sheaves at the lower end of the digging ladder and terminate at anchors on each side of the dredge cut. During operation, the spuds are alternately lowered and the dredge rotated partially about the lowered spud. By alternately pulling on the wire ropes, the dredge is caused to swing back and forth at a slow speed, and so control the width of cut and rate of excavation.
The main pump on the barge produces a suction at the suction mouthpiece proximate the cutter so that material may be drawn upwardly into the pump on the barge. The material exits the pump and is discharged via a pipeline to a location suitably removed from the channel. For example, the material may be discharged onto land or into the water at a distance from the channel. The main pump is typically a standard centrifugal pump with a fluid inlet coaxial with the impeller axis, and a fluid outlet generally tangential with respect to the impeller. By way of illustration, such a pump for a relatively large dredge having a 24 inch diameter discharge pipe might have a 28 inch diameter suction pipe. The main pump impeller will have a minimum clearance between the vanes of approximately 13" by 17" for the passage of the dredged material entering the pump. This clearance is important as described below. The main pump impeller is rotated by an engine or electric motor supplying approximately 5,000 horsepower.
A variety of operating conditions severely compromise the efficiency and capacity of the dredge pump. For example, the dredge pump must accept not only mud, clay, sand, and gravel, but also large stones, pieces of water-logged wood, lengths of wire rope, rubber hose, automobile tires, bottles, tin cans, broken-up concrete, and the whole variety of trash material that finds its way into a navigation channel. This condition compels that there be a substantial clearance between the impeller vanes thus limiting the number of vanes, and also requires that there be a significant clearance between the vane tips and the periphery of the pump casing. This is in contrast to water pumps where the clearance between the vane tips and the pump casing is kept as small as possible at the point of discharge where the water exits from the pump chamber.
When an object enters the pump that is too large to pass through, the dredge must cease operations until the object is removed. In most cases it will be found that the object has jammed against the leading edges of the impeller vanes, and can be removed through a manhole located in the suction pipe immediately in front of the pump. Occasionally the pump will have to be disassembled in order to remove the object.
Another operating condition unique to suction dredging is that when dredging an area having a bottom characterized by gas deposits and the like, gas can become entrained in the fluid entering the inlet mouthpiece. This can cause the main pump to lose its prime, or at least cause it to operate in less than an optimal manner. Also, the phenomenon of cavitation wherein vapor bubbles form and subsequently collapse within the pump degrades performance and subjects the pump to a risk of damage. The avoidance of cavitation often entails running the pump more slowly than would otherwise be desirable or using an impeller with a smaller diameter.
Problems due to loss of prime and cavitation may be reduced if the fluid material being dredged is pumped to the main pump inlet at a positive pressure. It has also been found that the main pump operates more efficiently if the fluid at its inlet is at a positive pressure. Such a positive pressure allows the pump energy to be transferred to the fluid in the form of kinetic energy (velocity head) rather than being transformed into static pressure head. Thus the fluid exits the pump at a greater velocity, whereby discharge transport is facilitated.
One method of supplying this positive pressure at the pump inlet presently in use has been to provide a water-jet pump booster system which injects a high velocity stream of water into the suction pipe aft of the mouthpiece to add energy to the suction system. While not actually adding any energy to the pump itself, the provision of such a booster system does in some conditions allow the pump to operate more efficiently, the increase in efficiency more than offsetting the power required by the booster system. However, it will be immediately appreciated that the water-jet pump booster system has the disadvantage of requiring a greater main pump capacity due to the increased volume of fluid passing into the suction inlet. In cases where the main pump is already operating to capacity, the booster system actually degrades overall performance.
An alternate approach has been to provide a centrifugal pump on the ladder, such a pump being conveniently referred to as a "ladder pump." However, as described above, the construction of existing centrifugal dredge pumps is such that fluid passing therethrough undergoes one or more right angle bends. This introduces frictional losses which undermine the potential benefits. Moreover, the most advantageous location for the ladder pump is as close to the suction mouthpiece as is practical. However, the configuration of the conventional dredge pump with the right angle bends precludes locating the pump proximate the mouthpiece because the inlet pipe or outlet pipe would extend beyond the sides of the digging ladder. The operation of the dredge requires that the digging ladder have as compact configuration as possible consistent with accomodating the cutter, cutter shaft, the suction pipe and the ladder suspension rigging. Extending the width of the digging ladder will interfere with positioning the cutter in close-clearance situations, as for example dredging alongside a dock. While the ladder pump would preferably be located as near the suction mouthpiece as possible, the above-mentioned clearance problems tend to dictate a position generally near the top of the ladder. Even if placement near the suction mouthpiece is feasible, the relatively heavy weight of a conventional centrifugal pump puts a maximum strain on the suspension system. This may make it difficult to retrofit dredges with ladder pumps without costly modification.
Thus, while the aforementioned approaches to the problem of increasing main pump efficiency and capacity have provided benefits by way of increased production, they have been accompanied by offsetting detriments that have rendered their implementation less than ideal. Nevertheless, the disadvantages described above have been accepted as inevitable for those situations where the benefits outweigh the detriments.